Composition and possible origins of dark crater ejecta on Europa

The origins of low-albedo material on Europa's surface have been of interest since Voyager first returned close-up images of the icy moon. Material ejected from Io is known to contribute an exogenic flux of dark material to Europa's trailing hemisphere, and hydrated salt compounds concentrated within chaos terrain, ridges, and pits may be endogenous to the subsurface ocean. Many of Europa's impact craters also exhibit dark ejecta, the origins of which are unknown. Our study examines the ejecta of several large impact craters to determine possible origins for this dark material. We compared the dark material found in crater ejecta to other surface materials using Galileo Near-Infrared Mapping Spectrometer data to assess similarities in composition between ejecta material and other dark materials on Europa's surface. Our analysis shows that dark material found in crater ejecta has similar composition to other dark features on Europa and may be the result of comparable sources or alteration processes. We also considered dark impactors as sources for the dark ejecta material. Using crater scaling laws, we estimated the impactor size for each crater and determined the impactor's potential contribution of dark material to the ejecta. We then compared these quantities to those derived using a radiative transfer model and the measured reflectance values of each dark-ejecta crater. Our model results show that the lower albedo of the ejecta of these craters cannot be solely attributed to an intimate mixture of the impactor material with the target material. In contrast, modeled impact heating and vaporization suggest sufficient amounts of ice could be removed in order to explain the observed low-albedo patterns, if preexisting or impactor-derived dark material is just 0.1% more resistant to vaporization than the ice. Given the lack of spatial correlation, and the presence of similar-sized craters without dark ejecta, these results point to either localized differences in the concentration of dark non-ice materials in Europa's shell, or variations in impact velocities and geometries leading to differences in the amount of vaporized ejecta.